1. Field of the Invention
The present invention relates to acoustic wave devices, or more specifically, to acoustic wave devices including resonator structures each based on adjacent coupled resonators.
2. Description of the Related Art
There have been increasing demands for filters and duplexers able to function as an unbalanced-to-balanced transformer.
The stacked bulk acoustic wave (BAW) filter, which has resonator structures each based on stacked BAW resonators, carries out this transformation through vibrations in different modes. However, this type of filter is inferior in band-edge steepness.
As a solution to this, structures that connect a stacked BAW filter with monolayer BAW resonators have been proposed. The monolayer BAW resonators work in a single resonance mode and thereby ensure good filter steepness. A specific example is a circuit having a ladder filter and a stacked BAW filter connected in series.
For example, Lakin et al. introduced in their report a filter having a stacked BAW filter connected with monolayer BAW resonators in a circuit (see K. M. Lakin, C. W. Andrus, J. R. Belsick, K. T. McCarron and W. H. Thornhill, “Wide Bandwidth Thin Film BAW Filters”, Proceedings 2004 IEEE Ultrasonics Symposium), and Japanese Unexamined Patent Application Publication No. 11-88111 has disclosed a filter and a duplexer having a stacked BAW filter and monolayer BAW resonators connected in a circuit.
However, for example, a filter combining a ladder filter and a stacked BAW filter connected in series should have stacked BAW filters and monolayer BAW resonators whose structure as viewed along the thickness direction is different from that of the stacked BAW filter, and these resonators and BAW filter should be connected with each other. As a possible constitution of this type of filter, comparative example 1 is illustrated in FIGS. 1 and 2. FIG. 1 illustrates the circuit of a filter, and FIG. 2 illustrates its schematic cross-sectional view.
As illustrated in FIGS. 1 and 2, this circuit has balanced-signal input/output terminals 2a and 2b and an unbalanced-signal input/output terminal 4, and a stacked BAW filter 10x and monolayer BAW resonators 6, 7, and 8 are arranged therebetween. The stacked BAW filter 10x is a resonator structure and has two BAW resonators 11a and 11b having the same structure, in which a piezoelectric substance (13 or 15) is sandwiched between two electrodes (12a and 12b, or 14a and 14b). Coupled via an acoustic coupling layer 16, these two BAW resonators can resonate with each other. The monolayer BAW resonators 6, 7, and 8 also have the same structure, in which a piezoelectric substance (6x, 7x, or 8x) is sandwiched between two electrodes (6a and 6b, 7a and 7b, or 8a and 8b). These three monolayer BAW resonators can resonate independently of each other.
The circuit illustrated in FIGS. 1 and 2 is usually built on a single substrate. However, this usual way of circuit formation involves a costly process of making the stacked BAW filter 10x and the monolayer BAW resonators 6, 7, and 8 all have different frequencies and structures.
Stacked BAW filters alone can also make up a filter. A possible constitution of this type of filter is illustrated in FIGS. 3 and 4 (Comparative Example 2). FIG. 3 illustrates the circuit of a filter, and FIG. 4 illustrates its schematic cross-sectional view.
As illustrated in FIGS. 3 and 4, this circuit has terminals 2a and 2b and a terminal 4, and stacked BAW filters 10x and 20x are arranged therebetween. The stacked BAW filter 20x is a resonator structure and has the same structure as that of its counterpart 10x, and has two BAW resonators 21a and 21b, in which a piezoelectric substance (23 or 25) is sandwiched between two electrodes (22a and 22b, or 24a and 24b). Coupled via an acoustic coupling layer 26, these two BAW resonators can resonate with each other.
The filter circuit illustrated in FIGS. 3 and 4 can be formed in fewer steps because the layer formation process is common to both the stacked BAW filters 10x and 20x. 
This constitution, however, also has a problem in that the low band-edge steepness of the stacked BAW filters 10x and 20x leads to reduced overall steepness.